Nick Clegg has launched a Liberal Democrat manifesto which he described as "an insurance policy against a government lurching off to the extremes" after the May 7 General Election.

With neither Labour nor Conservatives making a breakthrough in the polls, Mr Clegg said the big question for voters is which party will join David Cameron or Ed Miliband in coalition.

Mr Clegg urged voters to consider "what will become of Britain" if Ukip's Nigel Farage or the Scottish National Party's Alex Salmond get into government, dragging the prime minister to right or left.

He pledged that Liberal Democrats would always act responsibly, fairly and "in the best interests of Britain" if they return to government as part of a coalition, promising: "We will add a heart to a Conservative government and we will add a brain to a Labour one."

The Lib Dem manifesto set out on its front page the party's five priority policies which will be "red lines" in any coalition negotiation and which Mr Clegg said could be summed up in the single word "opportunity".

The policies are: balancing the national budget in a fair way; guaranteed education funding "from cradle to college"; an increase to £12,500 in the income tax personal allowance; an £8 billion hike in NHS funding; and five green laws to protect the environment.

Launching the 33,000-world document in south-west London, Mr Clegg said: "This manifesto is a blueprint for a stronger economy and a fairer society.

"This manifesto is a plan to finish the job of balancing the books, and to do so fairly by protecting our schools, hospitals and public services.

"This manifesto is an insurance policy against a government lurching off to the extremes.

"At its heart is one word that is absolutely central to what Liberal Democrats believe: opportunity. No matter who you are, where you were born, what sexuality or religion you are or what colour your skin is, you should have the same opportunity to get on in life."

Looking ahead to the prospects for coalition negotiations if the May 7 poll fails to deliver a conclusive result, Mr Clegg said: "I'm not denying that either David Cameron or Ed Miliband will be prime minister. One of them will. But you know and they know that neither of them will win outright. Neither of them will have a majority in Parliament.

"So what really matters is who they will have by their side."

He added: "Someone is going to hold the balance of power on May 8 and it won't be David Cameron or Ed Miliband. But it could be Nigel Farage. It could be Alex Salmond. Or it could be me and the Liberal Democrats.

"So ask yourself this: Do you want Nigel Farage walking through the door of No 10? Do you want Alex Salmond sat at the cabinet table? Or do you want the Liberal Democrats?

"The Liberal Democrats will add a heart to a Conservative government and we will add a brain to a Labour one.

"We won't allow the Conservatives to cut too much and jeopardise our schools and hospitals. And we won't allow Labour to borrow too much and risk our economy again."

The Lib Dem platform was unveiled shortly before the launch of Ukip's manifesto of what Mr Farage described as "serious, fully costed policies" for people who "believe in Britain".

With the eurosceptic party sliding in the polls, Mr Farage issued a plea to former Conservative voters to back "the new party of defence" - suggesting he would demand military spending substantially beyond the Nato target of 2% of GDP as a price for Ukip support.

An in/out referendum on Britain's membership of the European Union "as soon as possible", a five-year ban on unskilled migrants coming in to the UK and a points-based system for others are among other key manifesto pledges, alongside boosts for first-time buyers, military veterans, small businesses and hospital patients.

The events completed the unveiling of all five national party manifestos over the past three days, following Labour's promise of a "budget responsibility lock" on Monday, the Green vision of a "society that works for the common good" and a Conservative offer of cash for the NHS and an extension of the Right to Buy on Tuesday.

Speaking at a Battersea art gallery in front of a montage of multi-coloured handprints, Mr Clegg warned that Ukip involvement in government alongside Mr Cameron would mean "our public services cut to the bone; our communities divided; our shared British values of decency, tolerance and generosity cast aside", while a Labour administration supported by the SNP would result in "our economy crippled by reckless borrowing; our children destined to pay for it for years to come; the future of our United Kingdom in the balance once again".

He warned: "The truth is a few hundred votes in a small number of seats could decide whether it is Liberal Democrat MPs, Ukip MPs or SNP MPs who the next prime minister will be forced to listen to. There is a very thin line between Britain being governed by a coalition with a conscience or a government with a grievance."

And he said: "That's why every vote for the Liberal Democrats matters. That's why every Liberal Democrat MP elected next month matters. Only the Liberal Democrats can make sure the next government keeps Britain on track. Every Liberal Democrat MP makes Labour's reckless borrowing less likely. Every Liberal Democrat MP makes George Osborne's ideological cuts less likely. And every Liberal Democrat MP is a barrier between Nigel Farage and Alex Salmond and the door to 10 Downing Street.

"Because the Liberal Democrats will always act responsibly. We will always act fairly. And we will always act in the best interests of the whole United Kingdom."

Mr Clegg said he would not recommend any coalition deal which did not include the Lib Dems' five front-page priorities, and promised the party would "fight tooth and nail" for their implementation once in government.

Chancellor Mr Osborne said a vote for the Lib Dems or Ukip would pave the way for Labour to form the next government, which would result in "chaos, job losses and cuts to family incomes".

Mr Osborne said: "Look there's a really clear choice in this election, you can either have the strong leadership of David Cameron and his plan for working people that is delivering for our whole UK.

"Or you can have Ed Miliband's weak leadership and economic chaos, and the job losses, and the cuts to family incomes that would result.

"And a vote for any of the alternatives to the Conservatives is a vote for that Ed Miliband government and their economic chaos.

"So I say let's stick with David Cameron's strong leadership and the plan that's working."

Lib Dem manifesto chief and education minister David Laws said: "It is a manifesto for government and not for opposition.

"It is not a manifesto for recharging batteries, we have done enough recharging batteries - we have done enough recharging of batteries over the last hundred or so years.

"We are serious about getting into government again to deliver the things that are in this manifesto."

He defended the party's refusal to set out "red lines" on policy, adding: "It would be crazy for any party, and I'm saying not just the Liberal Democrats but for the other parties, to start - before the votes have even been counted - some sort of process of drawing red lines around each and every one of these policies in the manifesto.

"It will be an insane way of managing a negotiation when we don't actually know what clout we will have in the next parliament."